High performance sports cars and other high performance vehicles equipped with relatively powerful, higher horsepower engines tend to sacrifice fuel economy for performance. Though it may seem counterintuitive, since the engines and manual transmissions of these vehicles are configured for performance, less than aggressive operation can significantly reduce fuel economy. To prevent such a reduction in fuel economy and to meet state and federal fuel economy and emissions regulations, vehicle manufacturers have implemented electronic shift control systems, such as those disclosed in U.S. Pat. Nos. 5,809,835 and 6,145,398, where vehicle operation is monitored and the driver is prevented from manually shifting into one or more gears that would undesirably further reduce fuel economy or otherwise impair engine performance.
One known type of electronic shift control system includes an electronic controller that monitors engine operation and driver input in a six-gear, high performance car and activates a skip shift device, typically a solenoid, that cooperates with a shift-gate and/or gear shift lever of the car's manual transmission during a skip shift so that the driver can only up-shift from first gear into a gear other than second gear when driving non-aggressively. For example, in one know skip shift implementation, the driver is prevented from up-shifting from first gear into second gear during a skip shift. In another implementation, the driver is prevented from up-shifting into second gear and third gear.
When the car begins moving in first gear, the controller of the electronic shift control system monitors engine speed, vehicle speed, manifold absolute pressure (MAP), gas pedal position and other parameters to determine whether the driver is driving non-aggressively enough such that activation of the skip shift device is required to cause a skip shift to occur. When such a skip shift occurs, the driver is locked out of shifting into at least one gear, typically second gear, such that the driver must up-shift into some other higher gear. Doing so helps maximize fuel economy and reduces emissions by preventing excessive engine revolutions that would have occurred if the driver would have been permitted to shift into the locked out gear.
In one known skip shift implementation, called computer assisted gear selection, the skip shift device is a solenoid that is energized when the car is in first gear, the throttle is at or below a certain percentage of wide open throttle, the car is traveling at a relatively low speed that falls within a predetermined non-aggressive driving range, and the temperature of the engine coolant is above a preset temperature. For example, in one known implementation, the controller will energize the skip shift solenoid when the throttle is at 35% or less, the car is traveling at a speed of between about 15 mph and about 20 mph, and the temperature of the engine coolant is greater than about 170° Fahrenheit. In another known implementation, throttle position can be no greater than about 25% or 26%. At such non-aggressive driving conditions, energization of the skip shift solenoid under this system allows the driver to shift out of first gear but prevents the driver from shifting into second gear to improve fuel efficiency. As previously mentioned, depending on the system configuration, the driver can be prevented from up-shifting from first gear into second gear and third gear.
Other parameters can also be used in addition to or in place of the aforementioned to determine when to request a skip shift. For example, in at least one other known implementation, gas pedal position or a percentage thereof can be used along with one or more of the aforementioned parameters in determining when the controller will request a skip shift. Where MAP is used, it can be adjusted to compensate for changes in barometric pressure. Finally, while specific percentages, speeds and temperatures are listed in the preceding paragraph, it should be recognized that extensive testing and experimentation is done to calibrate each parameter used to determine when a skip shift is requested based on the particular characteristics of the vehicle including, e.g. engine displacement, transmission, weight, traction, etc. in which the skip shift is implemented.
While such electronic shift control systems have worked well at maximizing fuel economy and minimizing emissions during non-aggressive driving conditions, the skip shift solenoid must be monitored to ensure it is working properly. To do so, a diagnostic is used to monitor skip shift solenoid operation and an On-Board Diagnostic Computer code (OBDC2) is generated should failure be detected. Depending on the configuration of the controller, generation of a skip shift failure OBDC2 code causes a check engine or other indicator to be displayed to the driver. In addition, this can cause the controller to stop energizing the skip shift solenoid during non-aggressive driving conditions. Unfortunately, such a diagnostic has been known to generate a skip shift solenoid failure OBDC2 code when there is no such failure. Conversely, there also have been occurrences where the skip shift solenoid has failed without any OBDC2 code being generated. As a result, this can lead the owner of the high performance vehicle to incur unnecessary diagnostic expenses and reduced fuel economy.
What is needed is a better diagnostic that more accurately and reliably determines when a skip shift device has actually failed. What is also needed is a more robust diagnostic that minimizes and preferably prevents the occurrence of false failures.